This invention generally relates to assemblies for holding material specimens, and more particularly relates to a specimen holder and mating receptacle holder arrangement for a plasma system in which a voltage is applied to the sample by either self-biasing or an external biasing system, and in which the imposed electric field at the sample due to this voltage is shaped by both the geometry of the specimen and the geometry of the holder in such a way as to cause low energy ions from the plasma to ion sputter material from the surface and near surface region at a low angle to improve the imaging of the sample in a transmission electron microscope (TEM), field ion microscope (FIM), position sensitive atom probe, or a three dimensional atom probe.
A critical requirement for high resolution transmission electron microscopy (TEM) and high resolution electron energy loss spectroscopy is that the sample should be very thin relative to traditionally prepared and examined TEM specimens. It is well known that the conventional methods of preparing TEM specimens produce artifacts that detract from the quality of high resolution images because of damage at the top and bottom of the sample. With the specimen thickness requirements of the highest resolution instruments that can achieve sub-angstrom resolution, the surface artifacts can become an appreciable thickness fraction of the sample under observation and prevent the achievement of the desired resolution. For example, mechanical polishing techniques to promote electron transparency, such as the tripod polishing technique or dimple grinding, will create surface scratches and subsurface damage in the form of amorphous regions and crystalline defects such as dislocations and dislocation loops and twinning, and can even cause re-crystallization or phase changes. Ion milling for short times by using an Ar ion beam at a low angle is used to remove this type of damage and provides samples that are representative of the original sample when viewed in the TEM. However, it has been shown that even with an ion energy as low as 3 keV and ion milling at an angle of 5°, an amorphous region as thick as 12 nm can be created on both the top and bottom surface of the TEM sample. Focused Ion Beam (FIB) preparation of TEM samples is even worse. The amorphous damage region for 30 keV Ga ions can be 20 nm thick or more on both surfaces of the TEM sample and it can implant Ga into the sample. One direct observation study of the amorphous damage from 30 keV Ga ions used to prepare a TEM sample of silicon showed that the amorphous damage in a wedge shaped sample is 23 nm and this limits the minimum practical thickness for an as-prepared FIB sample imaged in the TEM to about 60 nm. FIB instruments capable operating at low energy can reduce the amorphous region in at TEM sample to about 4 nm. Low angle, low energy Ar ion milling has been shown to reduce this layer to less than 1 nm when an angle of 5° from the surface and energy of 250 eV is used. For preparing FIM samples used in atom probe class instruments such as the position sensitive atom probe or a three dimensional atom probe, FIB has been employed to produce the conical shaped sample that is needed. Here too, the surface of the samples prepared by FIB has the problem of amorphization due to the beam. A pillar of material is often used and prepared by FIB for electron tomography in a scanning transmission electron microscope (STEM).
Plasma processing in commercially available plasma cleaners has been used to remove some of the amorphous damage from FIB samples but not in the manner that will be described in this application. Plasma cleaning of TEM samples and the TEM holder uses oxygen or an oxygen based gas mixture and is typically used to remove hydrocarbon contamination on the samples while they are positioned in the TEM holder assembly. For the removal of amorphous damage, typically Ar or another inert gas is used. Prior to discussing how the plasma processing has removed this amorphous damage in past studies and how the present method is different, a brief description of the plasma is necessary.
A plasma can be described as an ionized gaseous state in which ions, electrons, exited atoms, atomic neutrals, and molecules coexist. The presence of charged particles in the plasma is responsible for its conductivity, which is used to maintain the plasma condition through applied electromagnetic fields. In the majority of practical applications, the plasma generation process requires reduced gas pressures in a space between opposite electrodes and either a DC or an RF electromagnetic field imposed across this space. In these fields, electrons can easily be accelerated and their collisions with molecules and neutral atoms are the basis for the primary mechanism for gas molecule decomposition, excitation, and ionization. This creates chemically active neutrals and ions, which can be beneficially used for surface cleaning. In glow plasma discharges, the random kinetic energy of both neutrals and ions is below 0.1 eV and the plasma ionization rates are typically around 0.1-1%. Although such plasmas are sometimes referred to as cold, they do contain considerably higher energy electrons with energies of 1-3 eV, corresponding to an average temperature of about 24,000 K. Some of the hot electrons have energies of 15-20 eV, which exceed gas ionization potentials and provide gas ionization and excitation, which results in the visible glow of a plasma. From this plasma composition, there are three possible modes for using it in surface cleaning:
(i) Acceleration of positively charged ions with large masses towards the surfaces to be cleaned which causes mechanical sputtering of the contaminants;
(ii) Acceleration of negatively charged electrons towards the surface, with impact momentums insufficient for sputtering, but high enough for heating and thermal activation of surface contaminants; and
(iii) Creation of chemically active atoms and radicals to form volatile products with surface contaminants that can be pumped from the vacuum system. The thermalization and surface desorption of these volatile products are enhanced by the presence of the hot electrons in the cold plasmas.
While the first two methods are most efficient in terms of cleaning rates, they are also the most likely to cause irreversible surface modifications by ion implantation, radiation damage, or surface heating when the orientation of the sample is normal to the incident radiation. When the preservation of the surface is a major requirement, such as in sample preparation for surface analysis or TEM preparation, the third mode is more suitable. This method does not require high D.C. (direct current) acceleration voltages and consequently RF (radio frequency) systems are commonly chosen for plasma cleaning systems. It permits efficient cleaning of both conductive and insulating surfaces. For the removal of amorphous damage, the first method is most desirable if the plasma processing parameters can control the energy and angle of the ions. Important parameters in characterizing a plasma are its energy and its density. These are controlled by the gas pressure, the applied power of the electromagnetic fields, and the geometry of the discharge system.
There are two principal geometries for RF plasma cleaners. In the inductively-coupled geometry, an external RF coil surrounds a chamber wall made from an insulating material, such as a quartz tube. This part of the wall acts as an RF electrode, which is positively and negatively charged following the RF field oscillations. In this geometry, the other electrode can be a piece of grounded metal such as part of the vacuum chamber or an inserted electrode. The specimen to be cleaned can either be moved into the quartz portion of the chamber, where it is exposed to both charged and neutral plasma interaction, or it can be placed inside a grounded metal tube connected to the quartz tube, where only chemical interactions can take place, due to the much longer life of chemically active neutrals than charged particles. In the capacitively coupled geometry, a pair of inserted RF electrodes, one of which can be the grounded chamber and the other the active antenna. A sample to be cleaned is placed in the space between these electrodes, where it is exposed to both charged and neutral plasma components.
In a typical RF plasma generator, one electrode is grounded and has a large surface area, e.g. the chamber wall, while the other has a smaller area and is connected to the RF power supply via an impedance matching network. This smaller electrode develops an average negative DC self-biasing potential, USB, due to the different masses and, hence, response times of electrons and ions to the RF oscillations. This electrode is referred to as the cathode, and the chamber wall as the anode. The self-biasing potential of the cathode can be up to several hundred volts and is dropped across a dark cathode sheath, which separates the cathode surface from the glowing plasma. The gradient of this potential is the electric field that accelerates ions from the plasma towards the cathode surface and may cause sputtering if USB, is high enough. For cleaning procedures this is undesirable, since the sputtered cathode material will be deposited onto the surface being cleaned. This can be prevented either by keeping USB, below the sputtering threshold for the cathode material or by shielding the sample from sputtered material. For plasma cleaning in a capacitively coupled system, the sputtering effects from the antenna are avoided by restricting USB to below the sputtering threshold for the cathode material by restricting the RF power. The same is true for an inductively coupled system. In addition, in both types of systems, the sample can be moved into the non-plasma position where it is shielded from direct sputtering, as described above. This is sometimes referred to as down-stream processing.
In both geometries, the plasma can have its own potential, UP, with respect to ground, due to the difference in the mobility of electrons and ions. For practical purposes, a plasma floating potential, UF, is also important, which reflects the potential of an insulated probe inserted into the plasma with respect to UP. When a floating surface is inserted into the plasma it is bombarded with ions having kinetic energies of e(UP-UF), which could be within the range of 5 to 40 eV for commonly used RF plasmas. If the immersed surface is biased negatively, it will attract ions from the plasma. If the immersed surface is grounded, it acts as an additional anode. In typically used geometries, the total area of the anode is considerably larger than the area of the cathode and UP is positive with respect to the anode, causing ion bombardment. If the anode area is small compared to the cathode area, then the plasma potential is shifted towards negative values. However, this shift will always be below the gas ionization potential. The shift of the plasma potential also depends critically on the plasma parameters. The plasma potential will be shifted positively if the RF power is increased or if the pressure is reduced. As discussed above, both of these parameters increase the mobility of the electrons in the plasma. A higher plasma potential results in higher energy ions striking the surface of the grounded sample. The average temperature of the electrons in the plasma can be determined from the plasma potential, since eUp=kTe, where k is Boltzman's constant and Te is the average electron temperature. The described characteristic potentials of the plasmas can be obtained from voltage/current curves obtained using suitable electrostatic probes. For the commercial plasma cleaners that accept TEM specimen rods, it is possible to obtain representative values of these plasma potentials if an analytical TEM stage with an isolated tip for probe current measurements is used.
For the purpose of the removal of amorphous damage from TEM samples, the consideration of the relative values of the bias voltage on the antenna, USB, the floating potential UF, and the plasma potential, UP, are critically important. For minimizing the thickness of the surface damage region from mechanically polished, ion milled, and FIB prepared TEM samples with ions extracted from the plasma, both the energy and the angle that the ions strike the surface are critical parameters, as they are with the focused low energy ion milling approach. Because of these considerations, for the plasma processing approach outlined in this application, the orientation of the sample is also very important. The removal rate of material from a sample by sputtering is dependent on several parameters that include the ion angle, the material, the ion species, and its energy. As mentioned above, the energy of the ions must be above the materials threshold energy for sputtering to occur. In the plasma cleaner, the energy of the ions striking the sample is determined by the potential difference between the sample and the plasma potential. If the TEM sample is held in a TEM specimen holder, then the ions will strike the surface at normal incidence. Even at low energies, ions impinging normal to a surface will implant and cause sub-surface damage. The removal rate is also very dependent on ion angle. It has been shown that the best surface quality from a polishing standpoint is achieved at an incident angle relative to the surface normal greater than 85° and preferably at about 89° and at lower incident angles, surface roughening can occur.
The first description of a plasma cleaner affecting an FIB prepared sample was done with a capacitively coupled plasma cleaner supplied by South Bay Technology, Inc., the assignee of the present application. In this process, an FIB lift-out sample was held in a TEM holder and Ar was used as the gas because of the damage to the support grid if an oxygen plasma had been used. It was shown that a higher than normal operating power would thin a Zn sample. Since Zn has a high volatility, it is not surprising that it would show sputter removal at a relatively modest power level. Using Ar ions below 1000 eV, Zn and Cd, two similar metals, have at least five times higher sputter yields than any other metal. The orientation of the sample relative to the plasma was parallel so that ions struck the surface at normal incidence. Subsequent studies of plasma processing to thin FIB prepared samples utilize the same orientation. The phrase “plasma trimming” was coined to describe the removal of material from a TEM sample by an Ar plasma in a plasma cleaner. The assignee's experiments with biasing a 500 Å gold-coated Si sample to −133 V demonstrated that the normal incidence processing was very slow. However, this work did demonstrate that the coating near the edges of the sample was removed at a much higher rate. The higher removal rate is due to the increase in the electric field induced at the corners because of the small radius of curvature, since the electric field is proportional to the applied voltage and inversely proportional to the radius of curvature.